1. Field of the Invention
The present invention relates to a method and system for forming a stacked gate insulating film, and more particularly, to a method and system for forming a stacked gate insulating film, wherein a dielectric film is stacked on a silicon oxide film.
2. Description of the Related Art
As semiconductor integrated circuits are more highly integrated and more miniaturized, MOSFETs (Metal Oxide Semiconductor Field Effect Transistor) are more miniaturized as well. FIG. 8 illustrates an exemplary diagram of a MOSFET.
As described in FIG. 8, in a MOSFET 101, a gate electrode 104 is formed on a semiconductor wafer 102 via a gate oxide film 103. On the surface of the semiconductor wafer 102, a source 105 and a drain 106 are formed, by two of which the gate oxide film 103 is sandwiched. By applying a voltage to the gate electrode 104, an inversion layer is formed on the surface of the semiconductor wafer 102 below the gate oxide film 103, and a channel 107 connecting the source 105 with the drain 106 is generated. In the MOSFET 101, the voltage to be applied to the gate electrode 104 is controlled, thereby the flow direction of carriers flowing to the drain 106 is controlled while the source 105 serves as a carrier source.
It is desired that the gate oxide film 103 employed in the gate area of the MOSFET 101 is to be made thin along with the miniaturization of the MOSFET 101. However, if the gate oxide film 103 is simply formed thin, a current flows to the gate oxide film 103.
An insulating film whose dielectric constant is higher than that of a silicon oxide film, such as a stacked gate insulating film including a silicon nitride film stacked on the stacked gate insulating film, may be employed for the purpose of decreasing the leakage current in the structure where the reduced thickness of an oxide film is made small while the physical thickness of the film increases. In this case, it is important that the silicon oxide film is maintained with high quality that the value of interface trap density is low, for example. If the quality of the silicon oxide film is deteriorated, the characteristics of the MOSFET are deteriorated in that a threshold value voltage shifts and the mutual conductance thereof is small.
Generally, a semiconductor wafer is heat-oxidized, thereby a silicon oxide film is formed on the surface of the semiconductor wafer. During the heat oxidation process, the semiconductor wafer including a silicon substrate is arranged in a heat treatment apparatus. Then, the semiconductor wafer is heated up to a high temperature of, for example, 850xc2x0 C. by the heat treatment apparatus. After this, processing gas is conducted into the heat treatment apparatus so that the surface of the semiconductor wafer is heat-oxidized. As a result of this, the silicon oxide film having a predetermined thickness of, for example, 30 angstrom is formed on the semiconductor wafer.
However, in such a heat oxidation process carried out at a high temperature, the oxidation speed is quite fast, thus it is difficult to control the thickness of the silicon oxide film small like 10 angstrom or so. Therefore, the silicon oxide film is hardly made thin.
On the other hand, if the heat treatment is carried out at a lower temperature, the oxidation speed will be slower than the above case, and the silicon oxide film can be formed thin. However, in such a case, the quality of the silicon oxide film is deteriorated such that the interface trap density becomes high. If the quality of the silicon oxide film is low, some problems arise in that the threshold value voltage of to-be-formed transistors remarkably varies. This results in unstable operations of the transistors.
The present invention has been made in consideration of the above problems, and it is accordingly an object of the present invention to provide a method and system for forming a stacked gate insulating film including a high quality thin silicon oxide film and a high dielectric film.
In order to achieve the above-described object, according to the first aspect of the present invention, there is provided a method for forming a stacked gate insulating film comprising a silicon oxide film and a dielectric film, which is stacked on the silicon oxide film and whose dielectric constant is higher than a dielectric constant of the silicon oxide film, the method comprising:
a heat oxidation process of heating a semiconductor wafer, and heat-oxidizing a surface of the semiconductor wafer, thereby to form a silicon oxide film on the semiconductor wafer,
an etch back process of etching back the silicon oxide film so as to make the silicon oxide film thin; and
a dielectric film formation process of forming the dielectric film on the thin silicon oxide film.
According to this invention, after the silicon oxide film is formed as a result of the heat oxidation process, the film is etched back so as to be formed thin. Thus, there is no need to form a silicon oxide film at a low temperature. After the high quality silicon oxide film whose interface trap density is low is formed at a high temperature, the silicon oxide film can be formed thin after etched back. In this structure, it is also possible that a variation in the threshold voltage of to-be-finally formed transistors is controlled. Further, a high dielectric film is stacked on the thin silicon oxide film, achieving a reduction in leakage current.
It is preferable that a temperature at which the heat oxidation process is carried out is equal to or higher than 800xc2x0 C. Because the heat oxidation process is performed at a temperature equal to or higher than 800xc2x0 C., the high quality silicon oxide film whose interface trap density is low can be obtained. In addition, the silicon oxide film is etched back, thereby the film can be formed thin while maintaining the high quality thereof.
It is preferable that the heat oxidation process includes a process of forming the silicon oxide film in such a way that a value of an interface trap density is equal to or smaller than 5xc3x971010/cm2 eV. In the structure where the interface trap density of the silicon oxide film is low, the threshold value of to-be-formed MOS transistors can be controlled. In order to set the interface trap density at that value, it is effective that the heat oxidation process is performed at a temperature equal to or higher than 800xc2x0 C., as explained above.
The heat oxidation process may include a process of forming the silicon oxide film to a thickness which larger than 20 angstrom, thereby forming the silicon oxide film at a high temperature. This prevents lowering the quality of the silicon oxide film. Even if the thickness of the silicon oxide film is made small in the etch back process, the quality of the formed silicon oxide film does not become low.
The etch back process may include a process of etching back the silicon oxide film with hydrofluoric acid. In this case, the thickness of the thin silicon oxide film can be uniformly formed.
The etch back process may include a process of etching back the silicon oxide film with activated specifies wherein a mixed gas including fluorine, nitrogen and hydrogen is activated using plasma. In this case, the etching speed is not high, thus the thickness of the silicon oxide film can easily be controlled. Unlike a wet etching process, there is no need to perform processes of cleaning and drying the thin silicon oxide film.
The dielectric film may be a tantalum oxide film or a zirconium oxide film. In this structure, a stacked gate insulating film whose dielectric constant is high can be formed.
According to the second aspect of the present invention, there is provided a system for forming a stacked gate insulating film, comprising:
a heat treatment apparatus which forms a silicon oxide film by heat-oxidizing a semiconductor wafer;
an etching apparatus which etches back the silicon oxide film so as to make the silicon oxide film thin; and
a dielectric film formation apparatus which forms the dielectric film which is stacked on the thin silicon oxide film and whose dielectric constant is higher than a dielectric constant of the silicon oxide film.
In this structure, the semiconductor wafer is heat-oxidized by the heat treatment apparatus, and a silicon oxide film is formed thereon. The silicon oxide film is etched back by the etching apparatus, and is made thin. In doing this, the thin silicon oxide film is formed on the surface of the semiconductor wafer. A dielectric film whose dielectric constant is higher than that of the silicon oxide film is formed by the dielectric film formation apparatus, thereby forming a stacked gate insulating film whose dielectric constant is high.
The system for forming a stacked gate insulating film may included a multi-chamber system. The heat treatment apparatus, the etching apparatus and the dielectric film formation apparatus may be prepared respectively inside chambers of the multi-chamber system.
In this structure, the stacked gate insulating film can automatically and continuously be formed.